1. Field of the Invention
The present invention relates to a piezoelectric device, a dust removing apparatus, and an imaging apparatus which use an electrical-mechanical energy conversion element (representatively, piezoelectric element).
2. Description of the Related Art
A piezoelectric material is a substance which has a direct piezoelectric effect in which an electric polarization occurs when distortion is applied, and conversely, an converse piezoelectric effect in which distortion occurs when an electric field is applied. The piezoelectric material is used for piezoelectric devices such as various types of sensors, filters, and actuators by utilizing the property that such a reversible conversion between electrical energy and mechanical energy is possible. Major types of these piezoelectric materials which have been used for the conventional devices contain lead, and for example as a representative one, there is lead zirconate titanate obtained by solid solutions of PbTiO3 and PbZrO3 having ABO3 type perovskite structure. Because the lead zirconate titanate has superior piezoelectric characteristics, and good temperature characteristics, it is utilized in a wide variety of regions. However, in recent years, there has been increasing concern over adverse influence of lead on the human body. In each country, the use of lead for glass or high-temperature soldering begins to be restricted in accordance with RoHS Directive or the like. For this reason, even in piezoelectric materials used for various types of devices, non-lead materials which do not use lead are sought as an alternative to existing materials.
One of non-lead piezoelectric materials which are now under development is barium titanate (BaTiO3). The barium titanate has relatively high piezoelectric characteristics among non-lead piezoelectric materials, but there is a problem that crystal structure undergoes a phase transition from orthorhombic to tetragonal at close to room temperature, and causes marked change to the characteristics at product usage temperature region.
As one of measures for dropping an orthorhombic-tetragonal structure phase transition temperature (Tr) which exists at close to room temperature from a temperature close to the room temperature, a method for adding a minute amount of calcium titanate (CaTiO3) is taken. Japanese Patent Application Laid-Open 2009-215111 discusses that, when the minute amount of such the CaTiO3 is added, the phase transition temperature is sequentially shifted to low temperature side according to the additive amount of calcium titanate to barium titanate.
However, the above-described conventional example has the following issues. Specifically, even when the minute amount of calcium titanate is added to barium titanate as in the above-described conventional example, the phase transition temperature still exists at close to driving temperature of the device. Therefore, variation of the piezoelectric constant with temperature remains great as compared with the lead-based piezoelectric material. More specifically, as temperature changes from the room temperature to a low temperature, the piezoelectric constant increases. When such the piezoelectric material is applied to an actuator or the like, there arises a problem that as temperature changes from the room temperature to a low temperature, displacement increases. Therefore, conventionally in order to correct variation of displacement of the piezoelectric actuator with temperature, it is necessary to detect temperature or displacement, and to control the displacement based on driving frequency or applied voltage.